(a) Schematic of a droplet laser hosting arc-like (AL) modes. A molecular layer is deposited between the mirror and the droplet. The droplet-solid interfacial tension, the solid surface tension, and the droplet surface tension commonly determine the contact angle. (b) AL oscillation paths (top) and simulated electric field distributions of AL modes (below) under different contact angles. (c) Left: side-view profiles of droplets with varying contact angles. Right: optical images of the droplets after pumping. Yellow boxes, the laser emission regions. (d) Optical spectra of droplet resonators with different interfacial tensions. (e) Contact angles and spectrally integrated laser outputs as a function of different biomolecular concentration of bovine serum albumin (BSA) in droplets.